1. Field of the Invention
The present invention relates to a non-volatile semiconductor storage device with a multi-layer structure of laminated memory cells and a method of manufacturing the same.
2. Description of the Related Art
With the popularization of mobile devices as well as the increase in the amount of data to be processed, demand for small, portable and large capacity non-volatile memory devices has grown. For example, NAND-type flash memory constitutes a large market that has been used for many applications, including mobile phones, digital cameras, digital movie cameras, etc. However, the flash memory has a limited number of rewrite cycles due to degradation in characteristics associated with charge injection into recording layers. In addition, problems arise due to insufficient information write and read rate. Therefore, there is a demand for new non-volatile memory elements to replace the above-mentioned flash memory. As one example, non-volatile memory elements have attracted public attention that utilize variable resistance type memory cells where information is stored by changing resistance states in a reversible manner (e.g., Non-Patent Document 1: P. Vettiger, G. Cross, M. Despont, U. Drechsler, U. Durig, B. Gotsmann, W. Haberle, M. A. Lants, H. E. Rothuizen, R. Stutz and G. K. Binnig, IEEE Trans. Nanotechnology 1, 39 (2002); Non-Patent Document 2: P. Vettiger, T. Albrecht, M. Despont, U. Drechsler, U. Durig, B. Gotsmann, D. Jubin, W. Haberle, M. A. Lants, H. E. Rothuizen, R. Stutz, D. Wiesmann and G. K. Binnig, P. Bachtold, G. Cherubini, C. Hagleitner, T. Loeliger, A. Pantazi, H. Pozidis and E. Eleftheriou, in Technical Digest, IEDM03 pp. 763-766).
A variable resistance type memory cell generally includes a variable resistance layer and electrodes sandwiching the variable resistance layer. The variable resistance layer, which may take two or more different electrical resistance states, records the difference in resistance values in correspondence with data, by changing the resistance state of the variable resistance layer through application of certain threshold voltage, threshold current, and threshold charge between the electrodes. Furthermore, the variable resistance layer includes a feature that it may read such data without any destruction.
For example, metal oxide has been recently proposed as material of variable resistance layers, including multicomponent oxide such as nickel oxide (NiO) or strontium zirconium oxide (SrZrO3). The metal compounds are uniformly deposited on a lower electrode layer using physical or chemical schemes. An upper electrode is further laminated thereon. As a result, a memory cell structure is created. In general, while the metal oxide material can readily provide a uniform film property, it will be in an electrically high-resistance state just after deposition. As such, it requires an operation referred to as “forming”: high voltage is applied across the memory cell, thereby causing a large current to flow through the metal oxide. This large current creates a region where a local current easily flows within the metal oxide, i.e., a minute current path which is so-called “filament path”. As a result, the memory cell structure will be operable as a storage element. This operation will be referred to hereinafter as “forming”. After this forming process, applying a voltage greater than a certain threshold value causes a filament path to be cut off or restored repeatedly, changing the electrical resistance value of the corresponding memory cell. As a result, data is written to the memory cell. Note that if the applied voltage is not more than the threshold voltage, the resistance value remains unchanged, which value may then be read as information.
However, the voltage for use in a forming process with respect to a memory cell with a metal oxide film is larger than the threshold voltage required for normal variable resistance switching. Thus, a huge current will flow at the instant when resistance of a variable resistance element changes from a high resistance state to a low resistance state during the forming process. This excessive current may result in destruction of wirings, transistors, diodes, or the like. Therefore, it is desirable to perform the forming process with a low voltage.
In addition, it is necessary to fabricate a uniform filament path in a metal oxide film for providing stable elements' characteristics. Uniform current paths with suitable size should be formed at appropriate positions, for otherwise variations of the elements' characteristics cannot be reduced or operational errors would occur, which could reduce the yield.
To avoid any operational errors, additional circuits, referred to as redundant repair circuits, are required for isolating any faulty bits. However, the more the faulty bits, the larger the area to be reserved for redundant repair circuits, which would result in larger chip area, i.e., increased manufacturing costs.